The current invention relates to a seal in the exhaust section of a gas turbine. More specifically, the current invention relates to a seal for preventing the recirculation of hot gas through an annular cavity formed between an exhaust diffuser and an exhaust cylinder in a gas turbine.
In an axial flow gas turbine, the hot gas leaving the last row of turbine blades is directed through an exhaust diffuser, thereby increasing the pressure ratio across the turbine section of the gas turbine. The exhaust diffuser is formed by inner and outer flow liners disposed between an exhaust cylinder and a bearing housing. The flow liners serve to create a smooth flow path for the hot gas. They also act as a barrier which prevents the flow of hot gas directly over the exhaust cylinder and bearing housing, thereby preventing excessive temperatures and thermal stresses in these components.
There is an annular cavity between the outer flow liner and the exhaust cylinder. Since any flow of hot gas through this cavity would undesirably heatup the exhaust cylinder, ideally this cavity is a dead air space. However, to allow for differential thermal axial expansion, there is a gap between the outer flow liner and the adjacent upstream and downstream components. This gap creates a recirculation flow path for the hot gas through the annular cavity. In addition to causing excessive temperatures, thermal stresses, and distortion in the exhaust cylinder, such recirculation also upsets the aerodynamic performance of the diffuser.
In the past, the recirculation of gas through the annular cavity was prevented by a seal comprised of a plurality of steel plates bolted to both the downstream flange of the outer flow liner and the downstream flange of the exhaust cylinder and extending therebetween. By blocking the flow path through the annular cavity, the seal prevented recirculation of hot gas. Unfortunately, this seal is expensive and required a great number of man-hours to install, being comprised of over two hundred bolts, twenty-four retaining plates and six seal plates. Moreover, the differential thermal axial expansion between the outer flow liner and exhaust cylinder at each start-up of the gas turbine induced stresses in the seal plates which eventually caused them to crack by a fatigue mechanism.
Accordingly, it would be desirable to provide an exhaust diffuser recirculation seal which was relatively inexpensive, easy to install, and sufficiently flexible to withstand differential thermal expansion.